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Caption

Pulmonary arteries. Coloured frontal 3D magnetic resonance angiogram (MRA) scan of part of the heart and the pulmonary arteries. The left and right pulmonary arteries branch from the top of the heart from where they carry deoxygenated blood to the lungs. The arteries branch into progressively smaller vessels. The smallest vessels wrap around tiny air sacs (alveoli), which are the site of gaseous exchange. Oxygen enters the blood and carbon dioxide leaves. The oxygenated blood then travels through the pulmonary veins (not seen) back to the heart where it is pumped around the body.